Solid state ceramic microwave heating susceptor utilizing compositions with metal salt moderators

ABSTRACT

Disclosed are improved ceramic compositions which are useful in the formulation and fabrication of microwave susceptors for disposable packages for the microwave heating of food items and to such articles themselves. The compositions include certain metal salts as time/temperature profile moderators in addition to novel microwave absorbing materials and a binder. Certain metal salts can be used to dampen or lower the final temperatures reached upon microwave heating the ceramic compositions. Other metal salts can be used to increase or accelerate the final temperatures reached upon microwave heating. The microwave absorbing materials comprise ceramics with neutral lattice charges such as clays, talc, kaolin, silicates, aluminosilicates, sodium metasilicate, alumina and mixtures thereof. The compositions provide good heat generation and a predeterminable upper temperature limit. The materials are common and inexpensive.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application to U.S. Ser. No.056,201, filed June 1, 1987 entitled SOLID STATE CERAMIC MICROWAVEHEATING SUSCEPTOR COMPOSITIONS.

BACKGROUND OF THE INVENTION

1. The Technical Field

This invention relates generally to the art of the microwave heating byhigh frequency electromagnetic radiation or microwave energy. Moreparticularly, the present invention relates to ceramic compositionsuseful for fabrication in or into microwave heating susceptors,especially for disposable microwave packages for food products.

2. Background Art

The heating of food articles with microwave energy by consumers has nowbecome commonplace. Such microwave heating provides the advantages ofspeed and convenience. However, heating breaded food with microwavesoften gives them a soggy texture and fails to impart the desirablebrowning flavor and/or crispness of conventionally oven heated productsdue in part to retention of oil and moisture. Unfortunately, ifmicrowave heating is continued in an attempt to obtain a crisp exterior,the interior is generally overheated or overdone. Moreover, themicrowave fields in the ovens are uneven which can lead to unevenness orboth hot and cold spots within food items or packaged food items beingheated.

The prior art includes many attempts to overcome such disadvantageswhile attempting to retain the advantages of microwave heating. That is,the prior art includes attempts at providing browning or searing meansin addition to microwave heating. Basically, three approaches existwhether employing permanent dishes or disposable packages to providingmicrowave heating elements which provide such browning or searing andwhich elements are referred to herein and sometimes in the art asmicrowave heating susceptors. In the art, materials which are microwaveabsorptive are referred to as "lossy" while materials which are not arereferred to as "non-lossy" or, equivalently, merely "transparent."

The first approach is to include an electrically resistive film usuallyquite thin, e.g., 0.00001 to 0.00002 cm., applied to the surface of anon-conductor or non-lossy substrate. In the case of a permanent dish,the container is frequently ceramic while for a disposable package thesubstrate can be a polyester film. Heat is produced because of the I² Ror resistive loss (see for example, U.S. Pat. Nos. 3,853,612, 3,705,054,3,922,452 and 3,783,220). Examples of disposable packaging materialsinclude metallized films such as described in U.S. Pat. Nos. 4,594,492,4,592,914, 4,590,349, 4,267,420 and 4,230,924.

A second category of microwave absorbing materials comprise electricconductors such as parallel rods, cups or strips which function toproduce an intense fringing electric field pattern that causes surfaceheating in an adjacent food. Examples include U.S. Pat. Nos. 2,540,036,3,271,552, 3,591,751, 3,857,009, 3,946,187 and 3,946,188. Such anapproach is only taken with reusable utensils or dishes.

A third approach is to form articles from a mass or bed of particlesthat become hot in bulk when exposed to microwave energy. The microwaveabsorbing substance can be composed of ferrites, carbon particles, etc.Examples of such compositions or articles prepared therefrom include,for example, U.S. Pat. Nos. 2,582,174, 2,830,162 and 4,190,757.

A review of the prior art, especially that art directed towardsprovision of heating susceptors for disposable packages for microwaveheating of foods indicates at least three basic problems exist in theformulation and fabrication of heating susceptors. One difficulty withthe third category of materials, generally, is that they can exhibitrunaway heating, that is, upon further microwave heating theirtemperature continues to increase. Great care must be taken infabrication of safe articles containing such materials. Metallized filmmaterials of the first category can be formulated and fabricated suchthat they do not exhibit runaway heating. However, such films sufferfrom the second problem; namely that while their operating temperaturesare quite hot, are at controlled temperature, and are sufficient tobrown the surface of nearby food items, due to their thinness and littlemass, only small quantities of heat are actually generated. Suchmaterials are thus unsuitable for certain foods which require absorptionof great amounts of heat in their preparation, e.g., cake batters. Thethird general problem is one of cost. Microwave susceptors frequentlycomprise costly materials. Also, fabrication of susceptor structuresfrequently is complex and expensive.

Accordingly, in view of the above-noted problems with present microwavesusceptors, an object of the present invention is to provide a devicewhich will heat under the influence of the microwave radiation up to anupper temperature limit at which temperatures the device comes to asteady state absorption of the microwave energy and heating to a highertemperature is precluded.

Another object of the present invention is to provide a heating devicewhich is disposable and adapted for use with pre-prepared foods.

A still further object of the present invention is to provide a heatingdevice which can be utilized as a non-disposable utensil.

A still further object of the present invention is to provide a heatingdevice which by appropriate selection of manufacturing parameters canprovide a predetermined upper temperature limit and moderate microwaveheating of the food item through absorption and moderation of themicrowave energy.

Another object of the present invention is to provide a heating devicewhich is inexpensive to manufacture, safe to use and well adapted forits intended use.

Surprisingly, the above objectives can be realized and new compositionsprovided which overcome the problems associated with previous materialswhich have been used for the fabrication of microwave heatingsusceptors. The present compositions do not exhibit runaway heating yetgenerate relatively large amounts of heat. Indeed, the final heatingtemperature can be controlled quite closely. Also, the presentcompositions are comprised of materials which are commonly available andinexpensive. In the most surprising aspect of the present invention, thecompositions comprise ceramic materials previously considered alone tobe microwave transparent.

Throughout the specification and claims, percentages are by weight andtemperatures in degrees Fahrenheit, unless otherwise indicated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a packaged food article for microwaveheating constructed in accordance with the teachings of the invention;

FIG. 2 is a perspective view of the packaged food article with outerpaperboard outerwrap opened and with an inner tray and sleeve showndisengaged;

FIG. 3 is a perspective view of the tray disengaged from the sleeve andholding several food pieces;

FIG. 4 is a perspective view of the tray with the food items removedshowing a microwave heating susceptor raised above its resting positionin the tray;

FIG. 5 and 5a are cross sectional views of the tray taken in thedirection of lines 5--5 of FIG. 3;

FIG. 6 is a perspective view of an alternate tray with a lid eachfabricated from the present compositions with food items removed;

FIG. 7 is a perspective view of the alternate tray taken in thedirection of lines 7--7 of FIG. 6; and

FIGS. 8-17 depict time/temperature response curves for ceramiccompositions exemplified in Examples 1-31.

SUMMARY OF THE INVENTION

The present invention provides compositions useful in the formulationand fabrication of microwave heating susceptors. The presentcompositions essentially comprise an active microwave absorbingmaterial, a metal salt temperature profile moderator and a binder. Inits article aspect, the present invention provides new and improvedmicrowave heat susceptors especially for packaged food items, topackages for such items and to the packaged food items themselves.

The present microwave absorbing materials are common ceramic materialshaving a neutral lattice charge. The microwave absorbing materials cancomprise from about 0.1% to 98% of the ceramic compositions.

The binder essentially comprises from about 2.0% to 99.9% of thecompositions. Conventional binder materials are suitable for use herein.

Useful metal salts for temperature profile modulators include sodiumchloride, magnesium chloride, sodium sulfate, zinc sulfate, calciumchloride, calcium oxide, lithium hydroxide, ammonium chloride, sodiumhydroxide, potassium hydroxide, sodium bicarbonate, potassiumbicarbonate, potassium bromate, ferric chloride, sodium chromate,lithium hypochlorite, sodium hypochlorite, potassium hypochlorite,titanium dioxide (rutile and anatase), sodium oxalate, ferrous ammoniumsulfate, boron suboxide, sodium metaborate and the like. The metal saltscomprise from about 0.1% to 10% of the ceramic compositions.

In its article aspect, the present invention resides in devicesfabricated from the present compositions. Such devices are microwaveheating susceptors generally in sheet form and which range in thicknessfrom about 0.5 to 8.0 mm. The susceptors find particular usefulness indisposable packages for the microwave heating of food.

In other article aspects, the present invention provides packagescomprising the present novel microwave susceptors and to packaged fooditems.

In its method aspect, the invention resides in novel heating methodsincluding subjecting the compositions to a microwave field while incontact with food items.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to compositions useful for fabricationinto heating susceptors such as are useful for disposable packages forthe microwave heating of food products. The compositions comprise adefined microwave absorbing material and a binder. In its articleaspect, the present invention resides in microwave heating susceptorsfor packaged food items, to packages for such items and to the packagedfood items themselves. The present compounds are an improvement in theceramic compositions described in co-pending, parent applicationentitled "Solid State Ceramic Microwave Heating Susceptor Compositions"(filed June 1, 1987 by J. Seaborne as U.S. Ser. No. 056,201, AttorneyDocket 4445) and which is incorporated herein by reference. Thecompositions there described essentially comprise a defined microwaveabsorbing material and a binder. Preferred compositions are described asadditionally comprising common salt.

In the present invention, it has been surprisingly discovered that othermetal salts can be used as temperature profile accelerators in additionto sodium chloride. Thus, ceramic compositions with improved or highertemperatures can be obtained through the addition of modest amounts ofcertain metal salts. Additionally, it has also been surprisinglydiscovered that certain other metal salts are useful for providing theconverse benefit; namely, can be used to provide ceramic compositionswith dampened or lowered temperature heating profiles. Collectively, thepresent dampeners, accelerators and super accelerators can be used aloneand in various combinations to control and to modulate the heatingprofile characteristics of microwave ceramic compositions.

In the ceramic industry, a distinction is made between "greenware," aceramic composition before firing or vitrification, and the finished,fired or vitrified ceramic compositions prepared therefrom. The firingstep profoundly changes a large number of properties of the ceramiccomposition's properties as the individual constituents are fused into ahomogeneous mass. Broadly speaking, the present invention is directedtoward compositions which would be considered greenware in the ceramicarts.

Certain of the microwave active materials have been used in greenwareceramic compositions, but generally at markedly different concentrationsand for different purposes than in the present invention. For example,kaolin reduces plasticity and tends to make the greenware mix short orlean. Likewise, alumina has a similar effect on the plasticity and willreduce green strength. Also, sodium metasilicate is not used at levelsgreater than 1% since greater amounts cause sticking and hinder moldrelease properties as well as decrease green strength.

The present materials and their other general properties are well knownand are described generally, for example, in "An Introduction to theRock Forming Materials," by Deer, Howie, and Zussman, Longman GroupLtd., Essex, England, 1966. or in "The Potter's Dictionary of Materialsand Techniques" by Frank and Janet Hamer, Watson-Guptill Publications(1986), each of which is incorporated herein by reference. Materials astherein described are generally and conventionally classified as orthoand ring silicates, chain silicates, sheet silicates, frameworksilicates and non-silicates. However, the materials useful herein canfall into any of these classifications although not all materials inthose classifications are useful herein.

As indicated above, the microwave absorbing materials useful hereinsurprisingly include a wide variety of ceramic materials previouslyregarded as microwave transparent. It is speculated herein that thesematerials have heretofore been unappreciated as being useful as consumermicrowave absorbing materials since most investigations of theirelectromagnetic interactions, i.e., absorption/transparency have beendone at very different frequencies or have been investigated as firedceramics. By ceramic materials are meant materials comprising oxygenattached to non-carbonaceous elements, and primarily to magnesium,calcium, iron, aluminum, silicon and mixtures thereof. The presentmaterials are further essentially characterized by a neutral latticecharge. "Neutral lattice charge" is used herein in its conventionalusage and means that the net relative electron surface charge densitiesof the material is essentially zero or that the cation exchangecapability is essentially zero for the constituent chemical make-up ofthe ceramic material. The present ceramic materials are furthercharacterized by relatively low electrical resistivity, i.e., about 0.1to 35 ohm.cm and are thus classifiable as semiconductors in the broadsense of the term.

Exemplary specific microwave absorptive materials include:

Sodium Metasilicate, Na₂ SiO₃ ;

Talc, Mg₃ [Si₄ O₁₀ ](OH)₂ ;

Kaolin, Al₄ ·[Si₄ O₁₀ ](OH)₈ ·4H₂ O;

Alumina and activated alumina, Al₂ O₃ ;

Clays (fine grained, natural, early argillareous materials);

Aluminosilicates; non-siliceous ceramics.

Of course, mixtures of these materials can also be used. Preferredmaterials include sodium aluminum silicate, sodium metasilicate, claysand kaolin and mixtures thereof due to the relatively flat or uniformityof their final heating temperature.

The present compositions include an effective amount of the abovedescribed microwave absorbing materials. The precise level will dependon a variety of factors including end use application, desired finaltemperature, and thickness of the susceptor to be fabricated from thepresent compositions. Good results are generally obtained when themicrowave absorbing material comprises from about 0.1% to about 98% byweight of the present ceramic compositions. Preferred compounds includefrom about 20% to 98% by weight of the microwave absorbing material. Forbest results, the ceramic compositions comprise about 40% to 98% byweight of the microwave absorbing materials. The particle size of themicrowave absorption material or refactory is not critical. However,finely ground materials are preferred inasmuch as the ceramic susceptorsproduced therefrom are smooth and uniform in texture.

Another essential component of the present ceramic compositions is aconventional ceramic binder. By the term "ceramic binder" is meant thatthe binder is capable of binding the present ceramic heating materialsinto a solid mass. The term is not meant to imply or require that thebinder material itself is necessarily ceramic in composition although itwell may be. Such ceramic binders are well known in the ceramic art andthe skilled artisan will have no problem selecting suitable bindermaterials for use herein. The function of the binder is to form theparticulate microwave absorbing material into a solid form or mass.Exemplary materials include both ceramic and plastic binders,respectively, such as cement, plaster of Paris, i.e., calcium sulphate,silica fiber, feldspar, pulverized Kelvar® (a polyamide fiber),colloidal silicas, fumed silicas, fiberglass, wood pulp, cotton fibers,and mixtures thereof. The binder can comprise from about 2% to 99.9% byweight of the present ceramic compounds, preferably from about 20% to80%. Exemplary, conventional plastic based binders, both thermoplasticand thermosetting, are described in U.S. Pat. No. 4,003,840 (issued Jan.18, 1977 to Ishino et al.) which is incorporated herein by reference.

In one preferred embodiment, the present compositions include binderswhich are organic thermoplastic resins especially those approved as foodpackaging material such as polyvinyl chloride, polyethylene, polyamides,polyesters, polycarbonates, polyimides, epoxies, etc. In theseembodiments, the thermoplastic resin binders can range from as little as20% up to 60% of the composition and preferably about 30% to 50%. Suchcompositions are especially well suited for fabrication into shapedmicrowave susceptors, especially food trays, e.g., for TV dinners orentrees.

The present ceramic compositions additionally essentially comprise atemperature profile modulator. Three subclasses of temperature profilemoderators exist: (1) dampeners, (2) accelerators or enhancers, and (3)super accelerators. Accelerators, for example, may increase thetemperature rate of increase with time when exposed to microwaveheating. Accelerators may also increase the maximum obtainabletemperature. Dampeners have the opposite affect while super acceleratorsexhibit a greater acceleration effect.

Exemplary useful dampeners are selected from the group consisting ofMgO, CaO, B₂ O₃, Group 1A alkali metal (Li, Na, K, Cs, etc.) compoundsof chlorates (LiClO₃, etc.), metaborates (LiBO₂, etc.), bromides (LiBr,etc.) benzoates (LiCO₂ C₆ H₅, etc.), dichromates (Li₂ Cr₂ O₇, etc.),also; all calcium salts, antimony chloride, ammonium chloride, cupricchloride, copper (II) sulfate (blue vitriol), magnesium chloride, zincsulfate, Tin (II) chloride, vanadyl sulfate, chromium chloride, cesiumchloride, cobalt chloride, nickel ammonium chloride, titanium dioxide(rutile and anatase), and mixtures thereof. Exemplary usefulaccelerators are selected from the group consisting of Group 1A alkalimetal (Li, Na, K, Cs, etc.) compounds of chlorides (LiCl, etc.),nitrites (LiNO₂, etc.), nitrates (LiNO₃, etc.), iodides (LiI, etc.),bromates (LiBrO₃, etc.), fluorides (LiF, etc.), carbonates (Li₂ CO₃,etc.), phosphates (Li₃ PO₄, etc.) sulfites (Li₂ SO₃, etc.), sulfides(LiS, etc.), hypophosphites (LiH₂ PO₂, etc.), also barium chloride,ferric chloride, sodium borate, magnesium sulfate, strontium chloride,ammonium hydroxide, Tin(IV) chloride, titanium (II) oxide, titanium(III) oxide, silver citrate and mixtures thereof. Exemplary useful superaccelerators are desirably selected from the group consisting of B₄ C(boron carbide), ReO₃ (rhenium (IV) oxide), Cuprous chloride, ferrousammonium sulfate, silver nitrate, Group 1A alkali metal (Li, Na, K, Cs,etc.) compounds of hydroxides (LiOH, etc.), hypochlorites (LiOCl, etc.),hypophosphates (Li₂ H₂ P₂ O₆, Na₄ P₂ O₆, etc.), bicarbonates (LiHCO₃,etc.), acetates (LiC₂ H₃ O₂, etc.), oxalates (Li₂ C₂ O₄, etc.), citrates(Li₃ C₆ H₅ O₇, etc.), chromate (Li₂ CrO₄, etc.), and sulfates (Li₂ SO₄,etc.), and mixtures thereof. Exemplary useful herein as accelerators arecertain highly ionic metal salts of sodium, lithium, magnesium, silver,barium, potassium, copper, iron, and titanium including, for example,sodium chloride, sodium sulfate, silver nitrate, silver citrate, sodiumbicarbonate, potassium bicarbonate, magnesium sulfate, sodium citrate,potassium acetate, barium chloride, potassium iodide, potassium bromate,copper (I) chloride, lithium chloride and ferric chloride. The mostpreferred accelerator useful herein is common salt due to its low costand availability. The temperature profile accelerator(s) can assist inreaching more quickly the final operating temperature of the ceramiccomposition. Also, the accelerator(s) increases modestly the finaloperating temperature of the ceramic composition.

The preferred ceramic compositions comprise from about 0.01% to about10% by weight of the metal salt moderators. Preferably, the presentcompounds comprise from about 0.1% to 6% of the moderator. For bestresults about 1% to 6% moderator is used.

While ceramic compositions can be formulated having higher amounts ofthese metal salt moderators, no advantage is derived therefrom. It isalso believed important that the temperature profile moderators exist inan ionized form in order to be functional. Thus, ceramic compositionsbeneficially containing these salts should contain some moisture at somepoint in the composition preparation.

The present ceramic compositions can be fabricated into useful microwaveheating susceptor articles by a simple admixture of the materials into ahomogeneous blend, and addition of sufficient amounts of water if neededto hydrate the binder. When plaster of Paris is used as the binder,typically, water will be added in a weight ratio to binder ranging fromabout 0.4 to 0.7:1. While the wet mixture is still soft, the ceramiccompositions can be fabricated into desirable shapes, sizes andthicknesses and thereafter allowed to harden or dry to a moisturecontent ranging from about 2.5% to 10%. Another common fabricationtechnique is referred to as compression molding. In compression moldinga damp mix, e.g., 3% to 10% moisture of water activated binders, areemployed, or a dry mix if not, is placed into a mold and subjected tocompression to effect a densification of the composition to form a firmbody. Still another useful fabrication technique is isostatic pressingwhich is similar to compression molding but with one side of the moldbeing flexible. Isostatic pressing is especially useful in formingcurved ceramic pieces.

The final heating temperature of the present compositions is mildlyinfluenced by the thickness of the susceptor elements fabricated. Goodresults are obtained when susceptor thickness ranges from about 0.5 to 8mm in thickness, both when using the present improved compositions andwhen using the previously described ceramic compositions without thetemperature profile moderators. Preferred susceptors have thicknessesranging from 0.7 to 4 mm. All manner of shapes and size heatingsusceptors can be fabricated although thin flat tiles are preferred insome applications.

Of course, one advantage of the present invention is that upon heatingin a conventional microwave oven, e.g., 2450 MHz, the ceramiccompositions will relatively quickly (e.g., within 30 to 300 seconds)heat to a final temperature ranging from about 300° to 600° F. whichtemperature range is very desirable in providing crisping and browningto foods adjacent thereto and consistent with safe operation of themicrowave oven.

Another advantage of the present ceramic compositions is that they canbe dried at temperatures above 180° F. (82° C.). Still another advantageof the present invention is that susceptors fabricated from the presentceramic compositions provide a microwave field modulating effect, i.e.,evening out peaks and nodes, i.e., standing wave points and, it isbelieved independent of wattage. This benefit is especially useful whensensitive foods such as cookie doughs or protein systems are beingmicrowave heated.

Still another advantage of the present ceramic compositions is that theyare believed to be useful not only with microwave ovens operating at2450 MHz but at all microwave frequencies, i.e., above as low as 300MHz.

Another advantage of the present invention is that the ceramiccompositions can absorb oil and/or moisture from portions, withoutsubstantial adverse affect on heating performance.

It is important that the susceptors fabricated herein be unvitrified,i.e., not subjected to a conventional firing operation generally above800° F. to 1000° F. (426° C. to 538° C.). Conventional firing can resultin a fused ceramic composition substantially transparent to microwaveand thus devoid of the desirable microwave reactive properties of thepresent invention.

The present ceramic compositions are useful in any number of microwaveabsorption applications. The present ceramic compositions areparticularly useful for fabrication into microwave susceptors which inturn are useful as components in packages for foods to be heated withmicrowaves.

In certain preferred embodiments, the ceramic compositions additionallyessentially include reinforcing fibers or fabric reinforcing. The fibersprovide additional strength and resistance from crumbling and breakage.Suitable fibers (natural or synthetic) (whether plate-like or rods) arecharacterized by possessing high aspect ratios (the ratio of the fiberswidth to its length) and in the case of fabric reinforcing are eithernonwoven, woven or of the cord variety. The fibers essentially comprisefrom about 0.5% to 20%, preferably about 1.0% to 5% of the ceramiccompositions.

Another advantage is that the ceramic susceptor can be coated withplastics or inorganic coatings to render the surface non-absorptive tomoisture and oil as well as providing a non-stick surface. Also,colorants, both organic and inorganic in nature may be incorporated atappropriate levels into either the coating or body of the ceramicsusceptor to aid in aesthetics without adversely affecting theperformance of the ceramic susceptor.

For example, FIG. 1 illustrates generally a packaged food item 10fabricated in accordance with the teachings of the present invention andsuitable for microwave heating. FIG. 2 shows that the article 10 canoptionally comprise a six-sided outerwrap 12 which can be plastic, paperor other conventional packaging material such as the paperboard packagedepicted. The article can further comprise an inner assembly 14 disposedwithin the outerwrap 12 which can comprise a sleeve 16 fabricated from adielectric material (e.g., cardboard, paper, polyester) and disposedtherein a tray 18. In conventional use, the consumer will open thearticle 12, remove and discard the overwrap 12, and insert the entireassembly into the microwave oven. The sleeve 16 is helpful although notessential not only to prevent splattering in the microwave oven, butalso to assist in securing the food items against excessive movementduring distribution.

In FIG. 2, it can be seen that the sleeve 16 can comprise an opposedpair of open ends, 20 and 22, an upper major surface or top wall 24, alower major surface or bottom wall 26 and an opposed pair of minor sideor wall surfaces 28 and 30. As can be seen in FIG. 3, the tray 18 holdsor contains one or more food items 32. FIG. 4 shows the tray 18 with thefood items 32 removed. Disposed within the tray 18 is one or moremicrowave heating susceptors such as microwave susceptor heating panel34. In this preferred embodiment, the susceptors are generally flat orplanar and range in thickness from 0.020 to 0.250 inch.

Still referring to FIGS. 3 and 4, with the cooking of certain foods, itmay be desirable to heat the food items 32 from only or primarily oneside by use of the heating susceptor panel 34 while at the same timeminimizing the heating of the food item 32 by exposing it to microwaveradiation through the walls of the package assembly 14. To allowmicrowave radiation to reach the susceptor 34, the bottom wall 26 ismicrowave transparent at least to the extent that sufficient microwaveenergy can enter the package to heat the susceptor 34. Side walls 28 and30 can each optionally be shielded with shielding 29 as can top wall 24thereby restricting the entry of microwave radiation through these wallsto the food product as is known in the art. The shielding 29 can be ofany suitable type material of which aluminum foil is a currentlypreferred material. With the use of shielding, the microwave radiationpenetrates the microwave transparent bottom 26 only. Accordingly,cooking of the food product 32 in this embodiment is accomplishedsubstantially totally by the heat transferred to the food product 32from the susceptor 34 although some microwave entry through the openends 20 and 2 occurs. It is pointed out that the terms microwavetransparent and microwave shield are relative terms as used herein andin the appended claims.

In FIG. 5, it can be seen that the heating panel 34 can optionallycomprise a thin finish layer 36, e.g., 0.00005 to 0.003 inch (0.001 to0.025 mm) to impart desirable surface properties, e.g., color, waterrepellency, smooth appearance, stick free, etc. In the simplest form,such a layer can comprise ordinary paraffin or a sodium silicatepolymerized with zinc oxide. The finish layer does not substantiallyadversely affect the performance of the microwave susceptor. Suchsurface property modification finds particular usefulness when themicrowave susceptors are used in medical settings. For example, it isknown to fabricate surgical implants, e.g., discs, cylinders, fromferrites which absorb microwave radiation to thermally treat tumors. Insuch applications wherein the present compositions are employed, waterrepellency may be particularly desirable.

Other types of packages can be utilized with the ceramic microwaveheater compositions of the present invention. It is an importantadvantage that the present compositions can be fabricated intosusceptors of different configurations whether regular, e.g.,corrugated, or irregular.

Another embodiment is depicted in FIG. 6. Thermoplastic resins arepreferred for use as the binder materials. In this embodiment, thearticle 10 in addition to outerwrap 12 as shown in FIG. 2 can comprise amicrowave heating susceptor 40 fabricated into trays, dishes or shallowpans whether square, rectangular, circular, oval, etc. which serve bothto contain and heat the food items. Such tray shaped susceptors 40 findparticular suitability for use in connection with a batter type fooditem 44, especially cake batters or with casseroles, baked beans,scalloped potatoes, etc. In one particular embodiment the tray 40 canadditionally include a cover 42 also fabricated from the present ceramiccompositions. Trays 40 with covers 2 are especially useful for batterfood items like brownies in which it is desired to form an upper or topskin to the food item 44.

In still another embodiment shown in FIG. 5A, the panel susceptor 34 canadditionally comprise a backing layer(s), especially a metal foil, e.g.,aluminum 46. The foil serves to reflect back to the susceptor 34microwave energy passing through the susceptor 34. The incorporation ofa microwave shielding or reflecting layer 29 in close proximity on theopposite surface of the ceramic susceptor 34 also serves to act as asusceptor temperature booster to elevate the operating temperaturesubstantially above the temperature obtained without a microwaveshielding or reflective layer 29. Final temperature reached can be ashigh as 100° F. or more over similar structures without the metal foil.Also, the use of the temperature booster can reduce the need for athicker ceramic susceptor to obtain the same temperature therebyreducing both production costs as well as final weights of the microwavepackage. Since the ceramic compositions adhere to the metal foil withsome difficulty and cause an in heating interference due toconductor-wave phenomena interaction, it is preferable to treat thesurface of the metal foil with an intermediate or primer layer (notshown) for better adherency, i.e., ordinary primer paints, or to have anintermediate silicone layer, paper layer or other polymer layer, or toselect those binders for the ceramic compositions with increasedcapacity to adhere to metal foils.

The skilled artisan will also appreciate that the present compositionsabsorb microwave radiation at a wide range of frequencies and not merelyat those licensed frequencies for consumer microwave ovens.

Other types of packages can be utilized with the heater of the presentinvention. It is an advantage that the present compositions can befabricated into different configurations whether regular, e.g.,corrugated, or irregular. The susceptor compounds of the presentinvention can also be utilized in non-disposable utensils adapted for alimited number of repetitive heating cycles by embedding the heater orotherwise associating the heater with a non-disposable utensil body. Thesusceptor is associated with the remainder of the utensil in a mannersuch that the heater will be in heat transfer relation to a product tobe heated in or on the utensil. The utensil can be in the form of anopen top dish, griddle or the like. However, the present compositionswill exhaust their ability to heat upon microwave exposure relativelyquickly, i.e., after only a few cycles of operation.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative and not limitative ofthe remainder of the disclosure whatsoever. It will be appreciated thatother modifications of the present invention, within the skill of thosein the food arts, can be undertaken without departing from the spiritand scope of this invention.

EXAMPLE 1

100 grams of calcined activated high alumina X-5111 (EnglehardCorporation, Edison, N.J. 08818) was dry blended with 40 grams ofmagnesium silicate (Ceramitalc HDT, R. T. Vanderbilt Company, Inc.,Norwalk, Conn. 06855). 7.5 grams of magnesium chloride M.W. 203.31 wasdissolved in 65 grams of distilled water and added to the dry mix. Theslurry was cast into 31/2 inch square tile frames 0.125 inches thick anda paper backing was bonded to the surface. The composite was dried at150° F. (65.6° C.) for 3 hours. The resulting tile was stable andexhibited minimal dimensional shrinkage. The structure weight was 28.8grams, density 1.147 g cm⁻³. The tile was subjected to a 750 watt, 2460MHz microwave field for a period of five minutes while the temperatureof the tile surface was monitored using a Luxtron 750® Fluoroptictemperature monitor equipped with ceramic clad fiber optic temperatureprobes and interfaced with an IBM PC/AT computer for data collection andhandling. The recorded and averaged temperature profile of the tile isshown in FIG. 8 as line 1. The metal salt in this example is acting as adampener on the susceptor.

EXAMPLE 2

Similar to Example 1 except with 7.5 grams of sodium chloride M.W. 58.44substituted for the 7.5 grams of magnesium chloride. The structureweight was 26.8 grams, density 1.068 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 8 as line 2. The metal salt in this example isacting as an accelerator on the susceptor.

EXAMPLE 3

Similar to Example 1 except with 7.5 grams of sodium sulfate M.W. 142.04substituted for the 7.5 grams of magnesium chloride. The structureweight was 26.5 grams, density 1.056 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 8 as line 3. The metal salt in this example isacting as a super accelerator on the susceptor.

EXAMPLE 4

5.0 grams of sodium metasilicate pentahydrate, 30.0 grams calciumsulfate hemihydrate, 10.0 grams of calcined activated high aluminaX-5111 (Englehard Corporation), 35.0 grams Kentucky Clay #6(Kentucky-Tennessee Clay Company, Mayfield, Ky.), 50.0 grams Hexafil--asemi-reinforcing clay (Hammill and Gillespie, Inc., Livingston, N.J.)and 7.5 grams of Goldart--Cedar Heights air floated secondary clay(Minnesota Clay, Bloomington, Minn.) were dry blended together to auniform consistency. 62 grams of distilled water containing 7.5 grams ofmagnesium oxide M.W. 40.31 was added to the dry powder mix and a pasteformed upon mixing. The paste was cast into 3.5 inch square tiles 0.125inches thick and a paper backing was bonded to the surface. Thecomposite susceptor was dried for 8 hours at 150° F. (65.6° C.). Theresulting tiles were intact and displayed a 23.4% shrinkage upon drying.The susceptor weight was 25.4 grams, density 1.012 g cm⁻³. The recordedand averaged temperature profile of the susceptor is shown in FIG. 9 asline 4. The metal salt in this example is acting as a dampener on thesusceptor.

EXAMPLE 5

Similar to Example 4 except with 7.5 grams of sodium chloride M.W. 58.44substituted for the 7.5 grams of magnesium oxide. The structure weightwas 32.6 grams, density 1.298 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 9 as line 5. The metal salt in this example isacting as an accelerator on the susceptor.

EXAMPLE 6

Similar to Example 4 except with 7.5 grams of sodium citrate M.W. 294.1substituted for the 7.5 grams of magnesium oxide. The structure weightwas 31.2 grams, density 1.243 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 9 as line 6. The metal salt in this example isacting as a super accelerator on the susceptor.

EXAMPLE 7

50 grams of sodium metasilicate pentahydrate, 30 grams of calciumsulfate hemihydrate, 10 grams of Hawthorn Bonding Fireclay (MinnesotaClay, Bloomington, Minn.) and 50 grams of sodium aluminum silicate weredry blended together to a uniform consistency. 70 grams of the dry mixwas added with stirring to 35 grams of distilled water containing 7.5grams of calcium chloride M.W. 110.99 and a paste formed upon mixing.The paste was cast into 3.5 inch square tiles 0.125 inches thick and apaper backing was bonded to the surface. The composite susceptor wasdried for 4 hours at 150° F. (65.6° C.). The resulting tiles were intactand displayed less than 2% shrinkage upon drying. The susceptor weightwas 29.6 grams, density 1.179 g cm⁻³. The recorded and averagedtemperature profile of the susceptor is shown in FIG. 10 as line 7. Themetal salt in this example is acting as a dampener on the susceptor.

EXAMPLE 8

Similar to Example 7 except with 7.5 grams of sodium chloride M.W. 58.44substituted for the 7.5 grams of calcium chloride. The structure weightwas 25.5 grams, density 1.016 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 10 as line 8. The metal salt in this exampleis acting as an accelerator on the susceptor.

EXAMPLE 9

Similar to Example 7 except with 7.5 grams of lithium hydroxide M.W.41.96 substituted for the 7.5 grams of calcium chloride. The structureweight was 29.4 grams, density 1.171 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 10 as line 9. The metal salt in this exampleis acting as a super accelerator on the susceptor.

EXAMPLE 10

10 grams of Tennessee Clay #6 (Kentucky-Tennessee Clay Company,Mayfield, KY), 10 grams of Hexafil, 10 grams of calcined activated highalumina X-5111, 10 grams of Hawthorn Bonding Fireclay (Minnesota Clay,Bloomington, Minn.), 10 grams A. P. Green Fireclay (Minnesota Clay), 10grams Goldart-Cedar Heights Clay (Minnesota Clay), 10 grams Yellow Banks401 (Minnesota Clay), Old Hickory Ball Clay (Minnesota Clay), 10 gramsNYTAL® Talc (R. T. Vanderbilt Company, Inc., Norwalk, CT 06855), 10grams of Cornwall Stone (Minnesota Clay), 10 grams Geistley Borate(Minnesota Clay), 20 gram sodium aluminum silicate and 20 grams ofFeldspar (Minnesota Clay) were dry blended together to a uniformconsistency. 70 grams of distilled water containing 7.5 grams of sodiummetaborate M.W. 137.86 was added to the dry powder mix and a pasteformed upon mixing. The paste was cast into 3.5 inch square tiles 0.125inches thick and a paper backing was bonded to the surface. Thecomposite susceptor was dried for 2 hours at 150° F. (65.6° C.). Theresulting tiles were intact and displayed less than 2% shrinkage upondrying. The susceptor weight was 28.8 grams, density 1.147 g cm⁻³ Therecorded and averaged temperature profile of the susceptor is shown inFIG. 11 as line 10. The metal salt in this example is acting as adampener on the susceptor.

EXAMPLE 11

Similar to Example 10 except with 7.5 grams of sodium Chloride M.W.58.44 substituted for the 7.5 grams of sodium metaborate. The structureweight was 32.6 grams, density 1.298 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 11 as line 11. The metal salt in this exampleis acting as an accelerator on the susceptor.

EXAMPLE 12

Similar to Example 10 except with 7.5 grams of sodium hydrogen carbonateM.W. 84.01 substituted for the 7.5 grams of sodium metaborate. Thestructure weight was 27.8 grams, density 1.108 g cm⁻³, thickness 0.125inches. The recorded and averaged temperature of the susceptor duringthe microwave exposure is shown in FIG. 11 as line 12. The metal salt inthis example is acting as a super accelerator on the susceptor.

EXAMPLE 13 10 grams of Tennessee Clay #6, 30 grams of Hawthorn BondingFireclay, 10 grams of A. P. Green Fireclay, 20 grams Goldart-CedarHeights Clay, 5 grams of Yellow Banks 401, 5 grams Old Hickory BallClay, 50 grams of Georgia Kaolin #6 Tile Clay (Georgia-Kaolin, Union,NJ) and 5 grams of Feldspar were dry blended together to a uniformconsistency. 70 grams of distilled water containing 7.5 grams of calciumoxide M.W. 56.08 was added to the dry powder mix and a paste formed uponmixing. The paste was cast into 3.5 inch square tiles 0.125 inches thickand a paper backing was bonded to the surface. The composite susceptorwas dried for 2 hours at 160° F. (71.1° C.). The resulting tiles wereintact and displayed less than 2% shrinkage upon drying. The susceptorweight was 29.2 grams, density 1.163 g cm⁻³. The recorded and averagedtemperature profile of the susceptor is shown in FIG. 12 as line 13. Themetal salt in this example is acting as a dampener on the susceptor.EXAMPLE 14

Similar to Example 13 except with 7.5 grams of sodium chloride M.W.58.44 substituted for the 7.5 grams of calcium oxide. The structureweight was 33.3 grams, density 1.326 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 12 as line 14. The metal salt in this exampleis acting as an accelerator on the susceptor.

EXAMPLE 15

Similar to Example 13 except with 7.5 grams of silver nitrate M.W.169.87 substituted for the 7.5 grams of calcium oxide. The structureweight was 30.6 grams, density 1.219 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 12 as line 15. The metal salt in this exampleis acting as a super accelerator on the susceptor.

EXAMPLE 16

15 grams of calcined activated high alumina X-5111 (EnglehardCorporation), 25 grams of Hawthorn Bonding Fireclay, 20 grams ofGoldart-Cedar Heights Clay, 10 grams of Yellow Banks 401, 25 gramsNYTAL® Talc (R. T. Vanderbilt Company, Inc.), 25 grams of Georgia Kaolin#6 Tile Clay (Georgia-Kaolin) and 15 grams of sodium aluminum silicatewere dry blended to a uniform consistancy. 70 grams of distilled watercontaining 7.5 grams of cobalt chloride M.W. 237.93 was added to theblend and a paste formed upon mixing. The paste was cast into 3.5 inchsquare tiles 0.125 inches thick and a paper backing was bonded to thesurface. The composite susceptor was dried for 1 hour at 180° F. (82.2°C.). The resulting tiles were intact and displayed less than 5%shrinkage upon drying. The susceptor weight was 29.6 grams, density1.179 g cm⁻³. The recorded and averaged temperature profile of thesusceptor is shown in FIG. 13 as line 16. The metal salt in this exampleis acting as a dampener on the susceptor.

EXAMPLE 17

Similar to Example 16 except with 7.5 grams of sodium chloride M.W.58.44 substituted for the 7.5 grams of cobalt chloride. The structureweight was 30.6 grams, density 1.219 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 13 as line 17. The metal salt in this exampleis acting as an accelerator on the susceptor.

EXAMPLE 18

Similar to Example 16 except with 7.5 grams of lithium hypochlorite M.W.58.39 substituted of the 7.5 grams of cobalt chloride. The structureweight was 30.3 grams, density 1.207 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 13 as line 18. The metal salt in this exampleis acting as a super accelerator on the susceptor.

EXAMPLE 19

100 grams of sodium metasilicate pentahydrate was mixed with 30 grams ofcalcium sulfate hemihydrate, 40 grams of Hawthorn Bonding Fireclay and40 grams of A. P. Green Fireclay. After blending to a uniform mix, 60grams of distilled water was added containing 7.5 grams of titaniumdioxide (rutile) M.W. 79.90. The resulting mix was plastic and easilyworkable. The mass was cast into 3.5 inch square tiles 0.125 inchesthick and a paper packing was bonded to the surface. The compositesusceptor was dried for 1 hour at 180° F. (82.2° C.). The resultingtiles were intact and displayed 5% shrinkage upon drying. The susceptorweight was 42.6 grams, density 1.897 g cm⁻³. The recorded and averagedtemperature profile of the susceptor is shown in FIG. 14 as line 19. Themetal salt in this example is acting as a dampener on the susceptor.

EXAMPLE 20

Similar to Example 19 except with 7.5 grams of sodium chloride M.W.58.44 substituted for the 7.5 grams of titanium dioxide (rutile). Thestructure weight was 42.0 grams, density 1.868 g cm⁻³, thickness 0.125inches. The recorded and averaged temperature of the susceptor duringthe microwave exposure is shown in FIG. 14 as line 20. The metal salt inthis example is acting as an accelerator on the susceptor.

EXAMPLE 21

Similar to Example 19 except with 7.5 grams of sodium oxalate M.W.134.00 substituted for the 7.5 grams of titanium dioxide (rutile). Thestructure weight was 39.4 grams, density 1.753 g cm⁻³, thickness 0.125inches. The recorded and averaged temperature of the susceptor duringthe microwave exposure is shown in FIG. 14 as line 21. The metal salt inthis example is acting as a super accelerator on the susceptor.

EXAMPLE 22

5.0 grams of sodium metasilicate, 30 grams calcium sulfate hemihydrate,15 grams of calcined activated high alumina X-5111 (EnglehardCorporation, 80 grams of Tennessee Clay #6 (Kentucky-Tennessee ClayCompany, Mayfield, KY) and 7.5 grams of Hawthorn Bonding Fireclay(Minnesota Clay, Bloomington, MN) were dry blended together to a uniformconsistency. 70 grams of distilled water containing 7.5 grams of boronoxide M. W. 69.62 was added to the dry powder mix and a paste formedupon mixing. The paste was cast into 3.5 inch square tiles 0.125 inchesthick and a paper backing was bonded to the surface. The compositesusceptor was dried for 2 hours at 170° F. (76.7° C.). The resultingtiles were intact and displayed less than 2% shrinkage upon drying. Thesusceptor weight was 31.7 grams, density 1.263 g cm⁻³. The recorded andaveraged temperature profile of the susceptor is shown in FIG. 15 asline 22. The metal salt in this example is acting as a dampener on thesusceptor.

EXAMPLE 23

Similar to Example 22 except with 7.5 grams of sodium chloride M.W.58.44 substituted for the 0.5 grams of boron oxide. The structure weightwas 29.2 grams, density 1.163 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 15 as line 23. The metal salt in this exampleis acting as an accelerator on the susceptor.

EXAMPLE 24

Similar to Example 22 except with 7.5 grams of lithium chloride M.W.42.39 substituted for the 7.5 grams of boron oxide. The structure weightwas 33.9 grams, density 1.350 g cm⁻³, thickness 0.125 inches. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 15 as line 24. The metal salt in this exampleis acting as a super accelerator on the susceptor.

EXAMPLE 25

Similar to Example 22 except that the sample thickness was 0.0625 inchesand major surfaces 3.5 by 3.5 inches. The structure weight was 14.2grams, density 1.132 g cm⁻³. The recorded and averaged temperature ofthe susceptor during the microwave exposure is shown in FIG. 16 as line25.

EXAMPLE 26

Similar to Example 22 except that the sample thickness was 0.0937 inchesand major surfaces 3.5 by 3.5 inches. The structure weight was 25.3grams, density 1.345 g cm⁻³. The recorded and averaged temperature ofthe susceptor during the microwave exposure is shown in FIG. 16 as line26.

EXAMPLE 27

Similar to Example 22 except that the sample thickness was 0.156 inchesand major surfaces 3.5 by 3.5 inches. The structure weight was 39.2grams, density 1.252 g cm⁻³. The recorded and averaged temperature ofthe susceptor during the microwave exposure is shown in FIG. 16 as line27.

EXAMPLE 28

Similar to Example 24 except that the sample thickness was 0.0625 inchesand major surfaces 3.5 by 3.5 inches. The structure weight was 13.3grams, density 1.060 g cm⁻³. The recorded and averaged temperature ofthe susceptor during the microwave exposure is shown in FIG. 17 as line28.

EXAMPLE 29

Similar to Example 24 except that the sample thickness was 0.0937 inchesand major surfaces 3.5 by 3.5 inches. The structure weight was 22.5grams, density 1.199 g cm⁻³. The recorded and averaged temperature ofthe susceptor during the microwave exposure is shown in FIG. 7 as line29.

EXAMPLE 30

Similar to Example 24 except that the sample thickness was 0.125 inchesand major surfaces 3.5 by 3.5 inches. The structure weight was 27.3grams, density 1.088 g cm⁻³. The recorded and averaged temperature ofthe susceptor during the microwave exposure is shown in FIG. 17 as line30.

EXAMPLE 31

Similar to Example 24 except that the sample thickness was 0.118 inchesand major surfaces 3.5 by 3.5 inches. The structure was compressed to adensity of 1.676 g cm⁻³ to remove as much void volume as possible. Therecorded and averaged temperature of the susceptor during the microwaveexposure is shown in FIG. 17 as line 31.

Examples 25-30 demonstrate the ability to control the temperature of thesusceptor through susceptor thickness. These examples also demonstratethat the effect of thickness is common to salts whether they aredampeners, accelerators or super accelerators. Example 31 demonstratesthe effect of densification of any susceptor to increase its heatingability in a microwave field.

What is claimed is:
 1. A package article for food to be heated bymicrowave energy in a microwave oven comprising:a tray for holding afood item having a top and bottom surface, a substantially planarmicrowave heating susceptor disposed within said tray, said microwaveheating susceptor fabricated from a ceramic composition, comprising: (a)a ceramic binder; (b) a ceramic susceptor material which absorbs energyand having a neutral lattice charge; and (c) a metal salt temperatureprofile moderator, wherein the composition is unvitrified, and whereinthe susceptor is in intimate physical contact with the food item andranges in thickness from about 0.3 to 8 mm.
 2. The article of claim 1wherein the binder comprises about 2% to 99.9% by weight of thecomposition and wherein the ceramic susceptor material comprises about0.1% to 98% of the composition.
 3. The composition of claim 2 whereinthe ceramic composition additionally comprises 0.1% to 10% of a metalsalt temperature profile moderator.
 4. The article of claim 3 whereinthe ceramic susceptor material is selected from the group consisting ofsodium metasilicate, talc, kaolin, calcined alumina, alumina oractivated alumina, clay, aluminosilicates, sodium aluminosilicates andmixtures thereof.
 5. The article of claim 4 wherein the binder isselected from the group consisting of calcium sulphate, cements,calcite, silica fiber, whether amorphorus or crystalline, dolomite,aragonite, feldspar, pulverized polyamide fibers, colloidal silicas,fumed silicas, fiberglass, wood pulp, cotton fibers, thermoplasticresins and thermosetting resins.
 6. The article of claim 5 additionallycomprising:a sleeve fabricated from a dielectric material having a topmajor surface, a bottom major surface spaced apart and parallel to thetop surface, a pair of spaced, parallel walls and a pair of spaced,opposite side openings and wherein disposed within which sleeve is thetray.
 7. The article of claim 6 wherein the tray comprises a tray bottomwall and a side wall,and wherein the susceptor conforms to the shape ofthe tray bottom wall and is disposed above the tray bottom wall.
 8. Thearticle of claim 7 additionally comprising a food item disposed withinthe tray on top of the susceptor.
 9. The article of claim 8 wherein thearticle additionally comprises a second heating susceptor disposedwithin the tray spaced apart and parallel to the first susceptor, saidsecond susceptor overlaying the food item and in physical contacttherewith.
 10. The article of claim 7 or 8 wherein the tray is circular.11. The article of claim 7 or 8 wherein the tray includes a plurality ofside walls at least two of which are parallel and of equal size andwherein the first and second susceptors are compositionally similar. 12.A package article for food to be heated in a microwave oven,a microwaveheating susceptor in the form of a tray for holding a food item; whereinthe susceptor is capable of heating in a microwave oven, and whereinsaid susceptor is fabricated from a ceramic composition, comprising: (a)a ceramic binder; (b) a ceramic susceptor material which absorbs energyand having a neutral lattice charge; and (c) a metal salt temperatureprofile moderator, wherein the compound is unvitrified.
 13. The articleof claim 12 wherein the binder comprises about 2% to 99.9% by weight ofthe composition and wherein the ceramic susceptor material comprisesabout 0.1% to 98% of the composition.
 14. The composition of claim 13wherein the ceramic composition additionally comprises 0.1% to 10% of ametal salt temperature profile moderator.
 15. The article of claim 14wherein the ceramic susceptor material is selected from the groupconsisting of sodium metasilicate, talc, kaolin, calcined alumina,alumina or activated alumina, clay, aluminosilicates, sodiumaluminosilicates and mixtures thereof.
 16. The article of claim 15wherein the binder is selected from the group consisting of calciumsulphate, cements, dolomite, calcite, silica fiber, whether amorphorusor crystalline, aragonite, feldspar, pulverized polyamide fibers,colloidal silicas, fumed silicas, fiberglass, wood pulp, cotton fibers,thermoplastic resins and thermosetting resins.
 17. The article of claim16 additionally comprising:a sleeve fabricated from a dielectricmaterial having a top major surface, a bottom major surface paced apartand parallel to the top surface, a pair of paced, parallel walls and apair of spaced, opposite side openings and wherein disposed within whichsleeve is the tray.
 18. The article of claim 17 wherein the traycomprises a tray bottom wall and a side wall,and wherein the susceptorconforms to the shape of the tray bottom wall and is disposed above thetray bottom wall.
 19. The article of claim 9 or 18 additionallycomprising a food item disposed within the tray.